1. Field of the Invention
This invention relates generally to a process for bonding aluminum by a metal reflow process in a vacuum, and more particularly, to the use of cadmium as the bonding material.
2. Description of the Prior Art
Processes and methods for brazing, diffusion bonding and soldering are widely used to join aluminum alloy members. Furthermore, solder or brazing alloys and fluxes are chosen to accomplish this goal. Such processes or methods usually comprise providing on at least one mating surface a layer of the specific bonding material, putting the mating surfaces in contact, and applying the particular procedure which results in joining the members.
For example, one such method chosen by prior art is a eutectic diffusion bonding method such as disclosed in U.S. Pat. No. 3,180,022, incorporated herein by reference. By this method, bonding appears to take place by formation of a eutectic between the aluminum surfaces and the bonding material, which after eutectic formation diffuses away from the interface to leave an extremely fine juncture line. The bonding materials used in this method are copper, gold, silver, tin or zinc.
Another example is a vacuum brazing process such as disclosed in U.S. Pat. No. 3,197,151, incorporated herein by reference. This process comprises subjecting the aluminum members to be joined to the action of a reactive metal vapor in a high temperature, low pressure environment, where the brazing alloy is disposed between the surfaces to be joined. In the vacuum brazing of aluminum products, magnesium vapor has been used as the gettering agent.
Finally, cadmium-containing alloys have been used for soldering aluminum members. See for example U.S. Pat. No. 3,969,110, incorporated herein by reference.
The prior art, as exemplified above, describes processes and methods that are designed in accordance with the physical properties of aluminum, i.e., to achieve joints that have as high a shear strength as possible, while attempting also to reduce the corrosion rate. In particular, aluminum alloys have found application as a bonding agent in the fabrication of aluminum for radiator panels for use in orbiting satellites, and for high temperature reactors.
A basic problem in bonding aluminum sheets results from the oxide layer which covers the sheets. During rolling and processing of the aluminum sheets, the aluminum oxide outer layer and other impurities such as hydrocarbons, become imbedded in the sheets. This imbedded layer generally has a constant thickness and has different electrical properties than aluminum. In general, this layer is not chemically compatible with diffusion processes.
This imbedded layer (Beilby layer) presents an electrical problem when aluminum is used as a high frequency (600 MHz) waveguide. At high frequencies, the radiation penetration depth tends to be less than 0.0010. The layer is generally greater than 0.0010 inches in thickness so that at high frequencies the radiation does not penetrate past the layer.
As a result the layer must be removed to promote bonding, to improve electrical conductivity and to reduce electrical noise. The prior art technique of copper plating the oxide layer and bonding together the plated layers does not solve this problem for high frequency applications.
FIG. 1 illustrates a rectangular waveguide. Such waveguides are generally constructed of two halves joined at or near the high current point. FIG. 2 illustrates an equivalent circuit for a seam between the high voltage points. Capacitance C1 and C2 results from the different electrical properties of the aluminum, aluminum oxide and the bonding copper layer. Sensitive, high frequency antennas should have a sufficient magnification factor (Q) in order to be able to detect background noise. Capacitance C1 and C2 cause a reduced Q and, therefore, an unacceptable Q factor results.